Cell proliferation and programmed cell death play important roles in the growth and development of an organism. In proliferative diseases such as cancer, the processes of cell proliferation and/or programmed cell death are often perturbed. For example, a cancer cell may have unregulated cell division through either the overexpression of a positive regulator of the cell cycle or the loss of a negative regulator of the cell cycle, perhaps by mutation. Alternatively, a cancer cell may have lost the ability to undergo programmed cell death through the overexpression of a negative regulator of apoptosis. Hence, there is a need to develop new chemotherapeutic drugs that will restore the processes of checkpoint control and programmed cell death to cancerous cells.
One approach to the treatment of human cancers is to target a protein that is essential for cell cycle progression. In order for the cell cycle to proceed from one phase to the next, certain prerequisite events must be completed. There are checkpoints within the cell cycle that enforce the proper order of events and phases. One such checkpoint is the spindle checkpoint that occurs during the metaphase stage of mitosis. Small molecules that target proteins with essential functions in mitosis may initiate the spindle checkpoint to arrest cells in mitosis. Of the small molecules that arrest cells in mitosis, those which display anti-tumor activity in the clinic also induce apoptosis, the morphological changes associated with programmed cell death. An effective chemotherapeutic for the treatment of cancer may be one that induces checkpoint control and subsequent programmed cell death.
Most compounds known to cause mitotic arrest and apoptosis act as tubulin binding agents. These compounds alter the dynamic instability of microtubules and indirectly alter the function/structure of the mitotic spindle thereby causing mitotic arrest. Because most of these compounds target the tubulin protein, a component of all microtubules, they may also affect normal cellular processes in which microtubules have a role. Hence, a need exists for small molecules that specifically target proteins associated with proliferating cells, such as Eg5.
Eg5 is one of several kinesin-like motor proteins that are localized to the mitotic spindle and known to be required for formation and/or function of the bipolar mitotic spindle. Recently, there was a report of a small molecule that disturbs bipolarity of the mitotic spindle (Mayer, T. U. et. al. 1999. Science 286(5441) 971–4). More specifically, the small molecule induced the formation of an aberrant mitotic spindle wherein a monoastral array of microtubules emanated from a central pair of centrosomes, with chromosomes attached to the distal ends of the microtubules. The small molecule was dubbed “monastrol” after the monoastral array. This monoastral array phenotype had been previously observed in mitotic cells that were immunodepleted of the Eg5 motor protein.
The distinctive monoastral array phenotype facilitated identification of monastrol as a potential inhibitor of Eg5. Indeed, monastrol was further shown to inhibit the Eg5 motor-driven motility of microtubules in an in vitro assay. Furthermore, monastrol had no apparent effect upon the related kinesin motor or upon the motor(s) responsible for golgi apparatus movement within the cell. Cells that display the monoastral array phenotype, either through immunodepletion of Eg5 or monastrol inhibition of Eg5, arrest in M-phase of the cell cycle. Unfortunately, however, the mitotic arrest induced by either of these mechanisms is transient. (Kapoor, 2000. J. Cell. Biol. 150(5) 975–80). Both the monoastral array phenotype and the monastrol induced cell cycle arrest in mitosis are reversible. Cells recover to form a normal bipolar mitotic spindle, to complete mitosis, and to proceed through the cell cycle and normal cell proliferation. This suggests that a small molecule inhibitor of Eg5 that induced a transient mitotic arrest may not be effective for the treatment of cancer cell proliferation. Nonetheless, the discovery that monastrol causes mitotic arrest is intriguing and hence there is a need to further study and identify compounds that can be used to modulate the Eg5 motor protein in a manner that would be effective in the treatment of human cancers. There is also a need to explore the use of these compounds in combination with other antineoplastic agents.